In electrophotography, photoconductive substances for photoreceptors include inorganic substances, such as selenium, cadmium sulfide, zinc oxide, and the like. It is, however, very difficult to form flexible electrophotographic photoreceptors by using these inorganic photoconductive substances. Therefore, organic photoconductive substances should be used for obtaining flexible electrophotographic photoreceptors. Such organic compounds conventionally proposed include poly-N-vinylcarbazole sensitized with pyrylium salt dyes as disclosed in Japanese patent publication No. 256258/73; photoreceptors comprising poly-N-vinyl-carbazole and 2,4,7-trinitrofluoren-9-one as disclosed in U.S. Pat. No. 3,484,237; arylamine photoconductive compounds sensitized with pyrylium salt dyes as disclosed in U.S. Pat. No. 3,141,770; photoreceptors mainly comprising an eutectic complex composed of a dye and a resin as disclosed in Japanese patent application (OPI) No. 10735/72 (the term "OPI" as used herein means "unexamined published patent application"); and the like. Although such photoreceptors possess excellent electrophotographic characteristics and are regarded to have a practical value, there still remain many problems in view of various performance requirements for photoreceptors. On the other hand, these photoreceptors, though depending on the end use or the method of production, generally show their excellent characteristics due to employing excellent photoconductive substances.
The inventors studied various photoconductive substances, and, as a result, found that certain kinds of hydrazone compounds are excellent as photoconductive substances for electrophotographic photoreceptors as described in Japanese Patent Publication No. 34099/85 and Japanese patent application (OPI) No. 163047/85.
It is also known that diarylamine compounds, particularly benzidine compounds, are excellent photoconductive substances for electrophotographic photoreceptors as taught in U.S. Pat. No. 3,265,495, Japanese patent publication No. 11546/64 and U.S. Pat. No. 4,265,990.
However, since the photoconductive substances, such as the above-described hydrazone compounds and benzidine compounds, are usually used in an amount of from 10 to 90% by weight, and preferably from 30 to 70% by weight, based on the total solid content of a photosensitive layer, when a photoreceptor is prepared by using only one kind of these compounds as a photoconductive substance, the hydrazone compound or benzidine compound is gradually crystallized in the photosensitive layer, to ultimately precipitate on the surface during long-term preservation or preservation under a high temperature condition, though such does not occur immediately after preparation. A photoreceptor having undergone such crystallization is of no practical use due to deterioration image quality, such as an uneven image, formation of white spots, and the like, upon toner image formation. Moreover, if the crystallization proceeds after formation of a toner image, crystals precipitate on image areas or the background to form stains, thus resulting in poor image preservability.
The above-described problem becomes more serious in particular cases where the photoreceptor is a transparent electrophotographic photoreceptor and is applied to use in which an image is required to be preserved for a very long period of time and is reproduced by an enlarging projection, such as use as a microfilm. Through enlargement of an image by projection, the crystals precipitated on the photoreceptor, even though being very minute, greatly reduce the image quality and reduce resolving power particularly required for microfilms, to make image information illegible, resulting in destruction of practical value.
When applied to microfilms, photoreceptors having formed images are expected to be preserved for more than 10 years, and are, therefore, strongly desired to have image stability sufficient for long-term preservation.